The tidal reach of many lowland river systems is extensive, often extending upriver 100 km or more. Cut-off meanders, backwater ponds, and blocked valley coves are all prominent features along this tidal reach. While significant progress has been made to understand sediment dynamics in similar off-river environments above the head of tides, less is known about the processes driving transport and sedimentation within these waterbodies when tidally influenced. To provide insight we evaluate the processes governing trends in deposition within off-river tidal waterbodies along the Lower Connecticut River. Sedimentation rates in these environments exhibit a clear seaward increase with growing tidal influence, with the flood-dominated asymmetry in tidal sediment flux (i.e. tidal pumping) via tidal channels the likely mechanism for trapping. The preferential trapping of fine-grained organic sediment in tidal backwater ponds and coves make these environments especially susceptible to the focused accumulation of sediment associated contaminants, with concentrations of sediment associated mercury an order of magnitude higher than neighboring floodplain environments. Once a tidal tie-channel is established (either naturally or by human intervention), the associated floodplain pond or cove quickly infills with sediment. In rivers like the Connecticut, the creation and routine maintenance of inlets connecting tidal floodplain waterbodies to the main channel have increased both the distribution and connectivity of these systems to the main river in recent centuries, thus enhancing sediment trapping along the tidal floodplain at an optimal time for capturing and storing legacy contaminants introduced during the industrial era.